1. Field of the Invention
This invention relates to fluid control systems and in particular to apparatus for indicating the condition of a fluid filter provided in such systems.
2. Description of the Prior Art
In U.S. Pat. No. 2,678,134 of Frank Middleton, owned by the assignee hereof, a filter indicating device is shown wherein a gauge is provided communicating the discharge from the filter with a vent passage to atmosphere. Valve means are provided for normally closing the vent passage and actuated by a predetermined differential fluid pressure across the filter to admit atmospheric pressure to the gauge thereby indicating the clogged condition of the filter.
In James E. Scheidt U.S. Pat. No. 3,358,835, owned by the assignee hereof, a filter condition indicator is shown to comprise a window in a tank associated with the filter system. A piston in the tank is visible through the window. Means are provided for communicating tank pressure and a low spring force to one end of the piston and for communicating return line pressure to the opposite end of the piston. The piston is provided with an indicating means which is viewable through the window when the piston is moved by an increase in the return line pressure to a preselected undesirable high level.
In U.S. Letters Patent of James L. Schmitt, U.S. Pat. No. 3,358,836, also owned by the assignee hereof, a filter element indicator is shown to comprise a dual reading filter condition indicator for indicating either a clogged or ruptured filter element. The indicator includes a body having transparent portions at opposite ends of a passage therethrough. A plunger means in the passage is selectively moved away from or toward the windows to provide the desired filter condition indication.
In U.S. Pat. No. 2,729,339 of Marion M. McCoy, a temperature responsive means is provided for detecting a clogged filter condition. The electrical circuit associated with the apparatus is operated only when the liquid flowing through the filter has been warmed to the usual engine operating temperature. The contact member comprises a bimetallic element which flattens out when heated to permit the clogged condition indication and which remains in a curved condition when unheated so as to prevent indication of the clogged filter element.